@tdizzel did a good job of explaining most of it. I would emphasize that many of the things that we think of as app updates, on iOS the only way to get an update is by getting an OS update.
Another thing to consider is that Google isn't writing an OS that is only supposed to work on Pixel phones and tablets. They're writing an OS that is supposed to work on "all teh things". Phones, not just Pixel, but Nexus and Galaxy and Fire and for dozens of manufacturers producing usually 4 to 20 phones per year each. Then tablets, same story. Then refrigerators, televisions, cars, printers, houses, robots and anything else you can think of that can run Android. Along with writing for a somewhat larger and more diversified audience, there are also, for obvious reasons, more partners in hardware - Qualcomm, Intel, MediaTek, all the OEM's, battery manufacturers, camera lens manufacturers, people who make the tiny ribbons that transmit data from board to the display, etc. Apple has 1 or 2 relationships to maintain per type of component, whereas Android as a whole will have a few or potentially even dozens or hundreds.
Qualcomm so far hasn't told Apple that they're going to stop supporting an 18 month old chip they made for Apple, which is probably built into Apple's contract for chips they bought from Qualcomm. They did do that to several OEM's who bought "off the shelf" chips recently. This is a reasons that the idea of Google trying to create their own chip could be a good thing - obviously it'd have to be well executed and competitive with what we see from Qualcomm, NVIDIA, etc - but it would give them power over their own destiny, enabling them to be able to promise longer periods of updates and security updates, should that be something they're interested in doing.